(I found that when searching how to use 4 big “hockey puk” SCRs to make a rectifier bridge)

Here is
a photo. As you can see, there are three sections. The bottom section,
which is the base of the cabinet, carries the 8 transformers. (Four are
visible.) The center section holds the cooling fans, the power
controls, and most of the wiring. The top section is the tool tray and
carrying handle. (I say ‘carrying handle’ a bit cautiously; this beast
weighs 140 pounds!) Scroll down to see the schematic and design notes!

Build your own arc welder!

Click on the picture to get a larger view. This is what most of you
wanted to see, so I have placed this image up here at the top of the
page. It is also included further down on the page where there is more
information on each component. Please note that this shematic is not
absolute. Semiconductor and inductor tolerances vary enough that you
will have to experiment with values and configurations to get it to
work in your own unique situation.

Why build your own welder?

With technology at virtually everybody’s fingertips, there is an
increasing opportunity to the home hobbyist. You likely are reading
this manual either knowing the potential of easy to find parts built
into simple designs or with a desire to know more about it. That’s what
this manual is about; my goal is to communicate these designs and allow
you as the reader to build useful tools and benefit not only from using
them, but from the knowlege and experience gained in actually planning,
assembling, and completing such a project.

Facts about homemade equipment

There are some important facts about homemade tools. You can’t always
save money by building your own equipment. Making your own tools can be
very time consuming. And homemade equipment isn’t always better than
storebought.

Here is the other side of these facts. Most of
us have more time than money. If we can find sources for cheap or free
parts, we can save a lot of money, time being the only other expense.
Also, some homemade tools are not even available in the store or may
have handy features that their storebought counterparts lack.

People build their own shop equipment for a variety of reasons, and some of these I’ve already hinted at:

They like to build things
They want to improve a design.
They need a tool that cant get any other way.
They need a tool to build another tool.
They want to save money.
Studying the Arc Welder

You don’t need to know how to weld to benefit from this manual. Even if
you know all about welding, what is inside a welder is a different
story . Before you can successfully build an arc welder, you need to
understand how they work and what components they use.

An arc
welder is a high amperage, low voltage power supply. There are two
types of these: constant amperage and constant voltage. The stick
welder is the constant amperage type. Wire feed welders are constant
voltage. Arc welders generally use transformers to reduce the voltage
and boost the amperage to levels useful for welding. TIG and other
types of welders use special high frequency power supplies which are
beyond the scope of this manual.

Laminated iron core
transformers have a constant amperage characteristic that makes them
ideal for welding. Inside of practically any welder there is a
transformer which consists of three basic parts: Primary windings,
secondary windings, and a laminated iron core. The windings are copper.
The primary windings connect to line voltages, and in welders this is
generally 240 volts. The secondary windings power the arc and are much
heavier copper windings. The windings are wound on the iron core. There
is no electrical connection between the primary and the secondary
windings. Electrical power is transferred magnetically throught the
iron core.

A welding power supply also needs a way to vary
the power to the arc. There are several ways to accomplish this. One
way is to have an incremental number of taps along the secondary
windings to draw various amounts of power from. Another is to configure
the transformer so that the primary winding may be moved toward or away
from the secondary, delivering more or less magnetic flux to the
secondary. Another is to vary the pulse width of the line current to
the primary winding. The welder in this manual uses the pulse width
type of controller.

Electrical schematic wiring diagram

Modifications to the welderYou can build the welder any way
you choose. A much simpler design would be to switch different
combinations of transformers on and off to give a variety of heat
settings. Or you could remove the end blocks of two transformers, put
them end to end, and configure a moving primary controller. The reason
I chose the pulse width controller for this manual is was that it
provides a simple reliable design with few moving parts.

Small 110-volt welder I made for my dad

The transformer and the heat selector are the basic building blocks of
an arc welder. There are, however, a number of other support components
that need mentioning. The cabinet that encloses the welder must be
designed to keep out welding dust. This cabinet assembly must include a
cooling fan to provide enough air flow to keep the components cool. A
ground clamp and electrode holder (often not included when you buy a
welder) are also needed before you can weld. And you need a 220 volt
receptacle to plug in your welder, and the cord and plug on the welder
itself.

Getting the parts

Part of the thrill in building the arc welder is obtaining and
modifying the components that make the power supply. The transformers,
cooling fans and parts of the cabinet come from old microwave ovens.

What I did was went around to the local appliance dealers and service
shops and told them what I wanted to do and they were happy to give me
their scrap microwave ovens. I also put an ad in the paper, because
most appliance retailers charge a fee to accept their customer’s old
appliance, and people were elated to bring their microwaves to me
knowing that I wouldn’t charge them to accept it and that it would be
recycled into a piece of homemade shop equipment.

One word of
warning, though. Your yard or garage will become cluttered with
microwave ovens awaiting dissasembly. You will need eight large
transformers to complete this project, and you will need microwaves
from 950 watts and up. If you advertise in the newspaper, you won’t be
able to choose what you get, but don’t despair; those odd ones may have
just the right transformer for your easy start sensor or just the right
fan for the cooling system. I counted a total of 22 ovens before my
welder was complete. I probably wouldn’t have needed that many, but I
got many good parts and probably enough transformers to build another
welder. At the time of this writing, I am toying with the idea of a
smaller welder that can operate on 120 volts for lighter projects.

The cabinet front and bottom are made from wood. The parts you will
need to buy are listed below. Most of these parts come from a hardware
store, except for the IRKT71 SCR module. This you will have to order
from an electronics supply company. I ordered mine from Newark
Electronics, but you may also find this part at Digikey Electronics, or
you can locate other sources on International Rectifier’s website.

Modifying the transformers

Microwave oven transformers are step up transformers. That means that
the voltage at the secondary winding is higher than the primary. In
microwave ovens, the primary accepts standard house current, 120 volts.
The secondary voltage is typically 4000 volts. The secondary winding
must be removed and a low voltage winding put in its place. The new
secondary winding has a typical open circuit voltage of 10 volts. Under
an arc welding load, this voltage will drop to between 2 – 4 volts, and
up to 250 amps. You will use #6 single conductor wire for the new
secondary winding. Many people ask exactly how many turns I put on this
new secondary, and I always say as many as you can fit! If you must
know, I got 12 to 15 turns on each transformer.

Mounting and wiring the transformers

Here are the details for the bottom board of the arc welder that the
transformers are mounted on. Since not all the transformers are the
same, you will have to improvise where needed. Mount the transformers
in such a way that the primaries and secondaries can be wired correctly
and neatly. You can even draw out the mounting patterns on the bottom
board to help organize it.

Building the cabinet

The cabinet design for the home built arc welder has several functions.
The top part resembles a tote tray and serves as a place to store
electrodes, welding gloves, cables and clamps, chipping hammers and
other items used in welding. The carrying handle is made of a 1 1/2
dowel, and provides a clue as to the weight of this machine.

The cabinet also functions as a chassis for the transformer and other
components. The cooling fans are mounted on the same plywood bulkhead
that the controller is built on. The transformers are mounted on the
bottom which is a short piece of pine 2×12. Building a sturdy cabinet
is imperative because the finished welder will weigh around 120 pounds.
Don’t skimp here.

You can paint the cabinet with any color
scheme you so desire, but the main purpose for the paint is to seal the
wood against moisture and solvents. It also gives the machine a
professional look that will bring a sense of worth to all your efforts.

The controller is the pulse width type. It works by energizing the
transformers with short bursts of current, medium bursts, or continuous
current depending on the setting on the heat selector dial, R1. It is
the same type of control circuit used in rotary light dimmers.

You can use predrilled perfboard, but I recommend building the phase
control circuit on an experimenter socket. It’s not that much more
expensive, and if a component blows, you can easily plug in a new one
without even having to warm up your soldering iron. Make extra sure
your connections are correct before you apply power, and never handle
the circuit with power applied!

For an SCR module I first
used two Teccor S6070W scr’s wired in an inverse parallel circuit, as
you see in the schematic. These proved to be too light duty and they
fried when I tried to weld at full heat with 5/32 rod. After carefully
comparing prices in several industrial electronics catalogs, I selected
International Rectifier’s IRKT71 Inta-pak SCR module. It costed about
$50 as I remember. I purchased it through Newark Electronics. Well
worth the price. It had 3 big screw terminals on top and 4 smaller
spade type connectors at one end for the control circuitry. It contains
two SCR’s internally and is configured with inverse parallel circuitry
in mind.

The SCR module and heatsink assembly must be
configured to receive a blast of air from one of the cooling fans. Use
heatsink grease between the SCR module and heatsink to provide a good
heat conducting contact. This assembly generally won’t be getting very
hot, and that’s just the point. Carefully make and verify the
connections to the phase control circuit, overheat sensor, and fan only
switch.

The easy arc starting circuit is optional. R2
controls the sensitivity. Adjust it to the least sensitive setting at
lowest heat. That way it is sure to work at all heat settings. It works
by putting full power to the electrode until you strike the arc. This
helps to keep the electrode from sticking to the work. Use a brain
board transformer from one of the ovens for this and modify it as
following: Identify and remove the secondary winding bobbin and run one
loop of #6 stranded cable through it. Connect the primary winding to
the indicated connections on BR1.

Tying up all the loose ends

This chapter deals with the final details needed to make your welder functional.

Do the final wiring according to the schematics. Connect the welding
cables and put on the ground clamp and electrode holder. Install the
range cord and wire it to the main power switch and transformer
terminal block. Attach the heat selector knob and you’re ready to plug
in your new arc welder.

Calibrating the heat selector dial
can be done any way you wish, it is not important to know the exact
amperages that are right for each welding application. I calibrated
mine with open circuit voltages, which when squared are roughly
proportional to welding current. To do this, set your voltmeter to a
scale suitable for 80 volts. Turn on the welder and disconnect the easy
start relay. Turn the heat selector knob to full power and mark the
spot on the dial. Then turn the knob back so that your voltmeter reads
70 volts and mark the spot on the dial. Turn the knob back to 60 and
mark the spot. Repeat this process at 10 volt increments. Or you can
increment it in 5 volts steps. If you can figure out a way to calibrate
the dial in amps using a very large ammeter, you can of course do that.

A crash course in welding

If you have never welded before, I recommend that you go to the library
and check out a manual on welding. If you MUST weld immediately after
finishing your welder, please read this chapter.

BEFORE you
strike an arc. It is important that you be properly dressed for
welding. You need a welding helmet to protect your eyes from the
ultraviolet rays and to keep sparks from getting into your hair. Fire
retardant hats are a good idea, too. You can get them from welding
supply houses. You also need gloves to protect your skin from arc
sunburn and welding spatter. Leather aprons and leather boots keep
welding spatter from getting to your skin. And remember to weld only in
well ventilated areas. Welding makes choking, dusty smoke. Read the
instructions and warnings on the labels of welding supplies and
equipment.

Striking and maintaining the arc. Striking an arc
isn’t complicated. With your helmet up, position your electrode about
1/4 inch away from where you want to start welding. Lower your helmet
and make a quick jab with the electrode. Be watching for the arc. Be
prepared to pull the electrode away SLIGHTLY. Very soon you will have
to slowly move the electrode into the weld as it melts off fairly
quickly into the weld pool.

Laying a bead. A properly
maintained arc makes a hissing, crackling sound as the electrode burns.
Holding the arc too far away makes more buzzing and spattering. Holding
the arc to close makes the rod overheat and sometimes stick to the
work. It is important in laying a bead to keep the electrode moving
into the weld pool as you move along. Horizontal beads are the easiest.
With vertical beads, it is the easiest to work from the top down. When
welding long beads, it is important to tack weld every 6″ to keep the
work from warping. For instance, if you are welding a metal box
together, tack the entire box together and then go back and lay the
beads solid. If you don’t, the whole mess will be so warped out of
shape after the first two seams you won’t be able to finish the other
welds.

Finally, remember that welding is something that takes
practice. You can’t learn it from a manual. You must spend some time
just laying beads and experimenting. Try welding bicycle frames. The
challenge here is to make nice welds without burning through the metal.
I’ve discovered you can cut bicycle frames and other thin metal
sections with a large welding rod on high amperage. Welding expertise,
however, is beyond the scope of this manual. Go to the library and get
some books on welding. Use them to guide your progress as you practice.

Troubleshooting

The welder seems stuck at high amperage and changing the heat selector
dial has no effect. There may be a number of things wrong here. Make
sure the easy start relay is wired properly. If this relay doesn’t pull
in when you strike the arc, the welder doesn’t switch down to the power
you select.
The arc is hard to start at low heat settings. The
easy start mechanism may be at fault. Make sure it is wired properly
and that the normally closed contacts are used. When you strike the
arc, the relay should open. You also have this problem if your welding
rod has its protective coating damaged at the striking end.

The
welder worked beautifully, but after welding 15 or so 5/32 rods, it
suddenly quit. You have overheated the welder. The overheat sensor has
done its job and shut down the controller. The fans should still run.
Let the welder cool off for a few minutes and it should weld again.

The welder worked beautifully, but after welding for two hours solid,
something smells funny and you either get no arc or only full amperage.
You have fried the SCR module and overheated the transformers. Most
welders have a duty cycle. That means that if your welder has an 80%
duty cycle, you must weld for no more than 8 minutes, and then let it
rest for 2 minutes before welding again. Or if it has a duty cycle of
30%, you must wait 7 minutes in between 3 minute welding spurts. A duty
cycle hasn’t been determined for this arc welder. It actually varies
depending on how high an amperage you are welding with. And remember to
keep the air vents open and to keep the insides of the welder free of
dust build up. Dust acts as insulation and hinders proper cooling.
Another thing to do to help the welder stay cool is to hit the fan only
switch between welds. This allows air to circulate the transformers
while they are idle.

I couple of years ago I bought one of welder pdf books and not I want to build a welder. I took so long because all the microwave transformers that I have different power outputs. Some I did not remember to keep the data sheet therefor their outputs are unknown.
In reading over and over the information in your book it says that the transformers have to be matched. As a result I have a few questions.
I presently have a 1000 watt, 1100 watt and 1500 watt.
My question is is it possible to build a welder with the 1000 watt and 1100 watt and as I go along collecting more add them in the circuit?
In rewinding these transformers can the number of turns on the secondary winding be adjusted so as to make the outputs closer.
For transformers that do not have the power output data is there some way of finding out?
On ebay I have seen many types of IRKT scrs module you specify – 71, 96, 105, 135, 210. Can I select any one of these or must it be the 71?

Please help me because a welder is expensive where I live on the island of St. Maarten, in the Caribbean.

I notice that the eight transformers have their secondaries wired in series. Does this add the voltages? Is the output voltage nominally 80v? You say this is a current transformer. I’m just learning about these things. But when you are not welding, this appears to be an open secondary circuit. I have heard you shouldn’t do that with a current transformer as the primary coils will build up a charge. Is this a danger when the unit is ‘on’, but you are not welding?

1. Any welder is an electrocution hazard when turned on, there is an open-circuit voltage there any time it’s turned on.

2. There is 80v AC present when not welding, this is referred to as “open circuit.” It does not continue to build up beyond that. This voltage varies from one welder to the next. I suppose my welder could be considered a little “high” on the open circuit. Find someone that has a welder and take your multimeter and test the open circuit.

Would this work on two separate 120v branches if you connected that center connection to neutral and put in a DPDT switch on the hot lines? The MOTs that I have seen are “class 220″ which I think means they are rated at 220 degrees which is a high rating… but still wouldn’t want to push them past their insulation rating on the primaries, or saturate the core.

I notice that the eight transformers have their secondaries wired in series. Does this add the voltages? Is the output voltage nominally 80v? You say this is a current transformer. I’m just learning about these things. But when you are not welding, this appears to be an open secondary circuit. I have heard you shouldn’t do that with a current transformer as the primary coils will build up a charge. Is this a danger when the unit is ‘on’, but you are not welding?

1. You can use any transformer you want that’s big enough, is practical to modify (i.e. split-bobbin as in mwt’s) and that you can get for free or very cheap, etc.

2. I don’t understand. Yes you have to connect an electrode for welding, before you can weld. Electrodes contain the material for welding. This seems obvious so I think there’s something I’m missing about the question.

3. Don’t worry, I was a beginner and didn’t even know how to weld when I made my first welder.

4. At least two transformers. One mwt probably does not have enough power. If you want to use only one, help yourself, I advise against it.

Hi, Dan. Great post. I was reading on a circuitry website for further info. The writer says that if you don’t tie the mid-points of the transformers together, you could have used mismatched transformers. I honestly don’t know. Any comment? Thx.

@Rhinodaddy, That’s correct. But I do tie the mid points together. Maybe I don’t need to. I think it was just to help prevent one side of the bank from over-volting the other side, to help ensure that they were balanced. Really only the two sides need to be balanced, and even whether you use 3 big ones on one side and 4 smaller ones on the other, would probably work, as long as the sides were equal. If not equal, then you might be giving a 120v transformer a big surge, maybe 180 volts and then it smokes.

Hi there, I recently purchased these plans and have a dozen microwaves in my garage, 10 shells and two more waiting for their untimely death. My problem is I’m having troubles finding enough #6 THHN wire at a reasonable price. Would it be possible / feasible to pair up the sheathed 14/2 or 12/2? What I mean is split open the Romex 14/2 and remove the ground and only use the two 14 guage hot and neutral together as ‘one’ conductor. Otherwise if I buy in spools of 100′, #6 THHN is 0.78c/ft. It drops down to roughly 0.57c/ft if I buy a 500 ft spool. Any help would be greatly appreciated!

@David, you certainly can use Romex or any wire you wish really. But I liked the THHN because it’s insulation was the least thick of about any, allowing more turns to be packed in. Also, being multi-stranded rather than a solid core wire made it easier to loop through the transformer’s core. Romex wire seemed too stiff. But what do I know.